A probabilistic look at the infinite hat-guessing game

Abstract: In this note, we look at a hat-guessing game, in which each player must guess the color of their own hat while only seeing the hats of the other players. We focus on the case of two hat colors and a countably infinite number of players. By strategizing in advance, the players can, in some ways, do much better than random guessing; using the axiom of choice, they can in fact achieve highly counter-intuitive success. We review some of these results. Then, we use tools from probability to obtain bounds on how successful a strategy can be under a measurability hypothesis, in terms of the asymptotic density of the set of correctly guessing players. As we discuss, this illustrates that the full axiom of choice is truly necessary for the counter-intuitively successful strategies, and that there is a wide gap between what can be achieved with and without choice.